Which issue(s) are closed by this pull request?

Closes #540, #542

Changes proposed in this pull request:

Change phase plots

• if unwrap='360': set y-range to 0-360 / 0-2 pi
• elif y-range smaller than pi: set range to +/- pi/2 with major ticks at pi/4, minor ticks at pi/8
• unwrap=True
  • If y-range is smaller than 20pi: major ticks at multiples of 2 pi, minor ticks at multiples of pi
  • If y-range is larger than 20pi: up to 10 major ticks, add multiples of multiples of 20pi, minor ticks at half of major ticks
• else (unwrap=False (default) or unwrap='360'): minor y-ticks at multiples of pi/4
• added missing tests for unwrap='360'
• added tests with small y-range (impulse signal)
• updated test plots

Have a look at the change of the baseline figures to get an idea.